Hemocyanin, tyrosinase and laccase are all copper-containing proteins which have a pair of contiguous Cu atoms important for biological activity. We propose to investigate mononuclear and binuclear Cu complexes as models for these proteins. Specifically we will 1) investigate five-coordination for Cu(1), 2) attempt to prepare stable dioxygen complexes from mononuclear and binuclear Cu(I), try to elucidate the mechanism of O2 activation and ligand oxidation via Cu(I), and 4) contrast the reactivity behavior of mononuclear vs. binuclear Cu(I) complexes.